You need the Flash player but it's pretty ubiqutous today. It may even come bundles with some browsers. If not, the first time you hit a page with an embedded .swf, you'll be prompted to download it (or update it as needed). Flash was originally devoted pretty much to doing vector graphics like the morphing shapes and letters you see on some pages. Over the years, it has evolved as Macromedia (now or soon to be part of Adobe) added new capabilities. Today, they're pushing Flash technology to create complete dynamic interfaces, much like DHTML or Javascript. I don't know how it compares for 3D with Viewpoint, Metastream etc. but for an installed base of players, Flash beats just about anything.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken